In recent years, to raise transfer efficiency, digital communication devices transfer information by modulating a carrier wave using a digital information signal (baseband signal).
In digital communication, frequencies can be efficiently used by raising the transfer speed and utilizing each single frequency band for multiple channels corresponding to multiple users.
The significant upsurge in the number of users of mobile communication services causes the shortage of frequency bands. The adaptive array method receives attention as a countermeasure against this problem.
The adaptive array method adaptively generates a directivity pattern using a plurality of antennas so that an adaptive array apparatus transmits radio waves only to a specific user during transmission and receives radio waves only from a specific user during reception. Take, for example, an adaptive array apparatus that has four communication subsystems that each include a transmission circuit, a reception circuit, and an antenna. The adaptive array apparatus respectively generates directivity patterns for transmission and reception by adjusting the amplitude and phase of the transmission signal in each communication subsystem during transmission and by adjusting the amplitude and phase of the reception signal in each communication subsystem during reception. The adaptive array method is described in Adaptive Signal Processing for Spatial Regions and Its Technical Applications (in “Transactions of the Institute of Electronics and Communication Engineers of Japan”, Vol. J75-B-II No. 11, November 1992) and so is not described in detail here. A Modified Adaptive Antenna Array Using Analytic Signal Processing (C. T. CHIANG; Y. H. CHEN, ANTENNAS AND PROPAGATION SOCIETY INTL SYMPOSIUM) discloses an adaptive array calculation technique in which desired signals are extracted by multiplying each input signal received via one of a plurality of antennas by a weighting factor.
A path division multiple access (PDMA) communication method is proposed which allows a plurality of users to simultaneously use the same frequency according to the adaptive array method. With the PDMA communication method, radio waves are transmitted and received using a plurality of directivity patterns which are each generated for one radio set (user). The PDMA communication method is described in Path Division Multiple Access (PDMA) Mobile Communication Method (in “Shingakugihou”, RSC93-84 (1994-01), Institute of Electronics and Communication Engineers of Japan) and so is not described in detail here.
When the adaptive array method is applied to a mobile communication, a directivity pattern needs to be adjusted according to the change in a propagation circumstance. This is because the location of a user changes and the propagation circumstance changes with time. When analog processing that uses a conventional phase unit is applied to such a mobile communication, various problems are created concerning the precision, stability, and trackability of the mobile communication. Therefore, it is realistic that the adaptive array method is realized using digital signal processing.
The amplitude and phase of the signal dealt with in each communication subsystem are adjusted using the digital signal processing, as described above. In the case of a digital quadrature phase modulation, for instance, appropriate weights are assigned to the inphase component and the quadrature component of each symbol dealt with in each communication subsystem. The inphase component and quadrature component of each symbol are hereinafter simply referred to as an I component and a Q component, respectively. By doing so, the amplitude and phase of each symbol are adjusted. Accordingly, the signal processing is mainly composed of calculating the weighting factors corresponding to the I and Q components dealt with in each communication subsystem.
To adjust a directivity pattern according to the change in a propagation circumstance, the time interval for the adjustment needs to be shorter than the time taken by the propagation circumstance to change. Therefore, for instance, it is preferable that the directivity pattern is adjusted in each symbol period. Also, the transfer speed of data in a recent mobile communication system becomes faster and the symbol period is shortened. In the PHS (personal handyphone system), for instance, the symbol period has been shortened to about five microseconds. Consequently, directivity patterns need to be adjusted in a short time.
The amount of signal processing necessary for the adaptive array method is significantly great because matrix calculations need to be performed to calculate the weighting factors described above. This causes a problem of how to perform a large amount of signal processing for the adaptive array method.
This problem becomes more pronounced when a plurality of users simultaneously use the same frequency according to the PDMA communication method to efficiently use frequencies. That is, the amount of signal processing increases in proportion to the number of users due to the signal processing needs to be performed for each user.
It is difficult for one DSP (digital signal processor) to simultaneously process signals for a plurality of users due to the limitation of its signal processing capacity. To cope with this problem, a plurality of DSPs may be used to perform signal processing necessary for the adjustment for a plurality of users in parallel. However, this inevitably increases the hardware scale and cost of an adaptive array apparatus.
In view of the stated problems, the present invention provides an adaptive array apparatus which reduces the amount of signal processing necessary for the adjustment of a directivity pattern and minimizes the increase in the hardware scale and cost.
More specifically, the present invention provides an adaptive array apparatus which appropriately performs the signal processing necessary for the adjustment for each user even if the number of users increases.